To print an image on an appropriate substrate, a typical digital printer first forms a copy of the image, conventionally referred to as a “latent image”, on a photosensitive surface of a cylindrical roller, hereinafter referred to as a “photosensitive imaging cylinder” (PIC). To form the latent image a charger deposits a substantially constant charge density on the “photosurface”. The latent image is then formed by a laser that scans the charged photosurface and discharges regions thereon to generate a pattern of charged and uncharged pixels on the photosurface that replicates the image to be printed. A developer applies toner, hereinafter generically, toner, of desired color to the charged or the uncharged pixels using an electrophoretic process to form a developed image.
In some digital printers, the developed image on the PIC is then transferred from the PIC to a suitable transfer surface of another roller, conventionally referred to as an “intermediate transfer member” (ITM). The developed image is transferred from the ITM to a substrate to print the image when the sheet passes through a nip between the ITM and an impression roller. The transfer surface of the ITM may be a surface of a removable printing blanket. In other printers, the developed image is transferred directly from the PIC to the substrate.
The amount of toner that is transferred from dots of toner on the photosurface of the PIC to the blanket of the ITM and the quality of the transfer is sensitive to an amount of pressure with which the PIC and ITM contact each other along their nip. In addition, the dot gain of the transfer is a function of the pressure. Pressure between the PIC and the ITM should be substantially the same for all regions of the nip and magnitude of the pressure should be such as to provide for proper printing press operation and quality of images printed by the press. If contact pressure between the PIC and ITM is too high, the image may be damaged during transfer, for example, by dot spreading or blurring of the image. Too much pressure also usually results in an increased wear rate for both the ITM blanket and PIC photosurface.
On the other hand, if contact pressure between the ITM and PIC is too low, too little toner may be transferred from the PIC to the ITM, resulting in colors in the image being washed out. If pressure is not substantially the same for all regions along the nip a printed image may be mottled, with some regions of the image receiving too much toner and other regions too little toner.
Generally, the surface of the ITM blanket is resilient so that spacing between the PIC and the ITM in a printing press controls pressure in the nip between them. As necessary, the spacing may be periodically adjusted to maintain printed image quality and proper printer operation. Pressure adjustment usually involves printing a suitable test pattern on a substrate and visually inspecting the printed test pattern to determine if the printed image looks good and, if the inspection warrants, manually adjusting the pressure. The process of printing and inspecting the test pattern and adjusting pressure is repeated until the pressure is deemed satisfactory. However, visual inspection and manual pressure adjustment are subjective, generally tedious and wasteful of press operator time.